[receiver/kafka] Add JSON-encoded log support

[Caleb-Hurshman]

Description:
Added support for json encoded logs to the kafkareceiver
Re-creation of pr #22982 after rebasing issues

Link to tracking Issue: #20734

Testing: Added tests for json_unmarshaler.go in json_unmarshaler_test.go, similar to existing 'unmarshaler' files.

Documentation: Updated README.md of the kafkareceiver to include the JSON encoding option.

[djaglowski]

kafkareceiver/unmarshaler.go:78: File is not `gci`-ed with --skip-generated -s standard -s default -s prefix(github.com/open-telemetry/opentelemetry-collector-contrib) (gci)
level=info msg="[runner] linters took 5.481112821s with stages: goanalysis_metalinter: 5.478905617s"
  json := newJSONLogsUnmarshaler()
kafkareceiver/unmarshaler.go:83: File is not `gci`-ed with --skip-generated -s standard -s default -s prefix(github.com/open-telemetry/opentelemetry-collector-contrib) (gci)
    json.Encoding(): json,
kafkareceiver/unmarshaler_test.go:53: File is not `gci`-ed with --skip-generated -s standard -s default -s prefix(github.com/open-telemetry/opentelemetry-collector-contrib) (gci)
    "json",

[djaglowski]

I believe all concerns mentioned on #22982 were resolved.